doya.hs

main = do
  print $ hoge ["abcdef", "abc123"]
  print $ hoge ["あいうえお", "あいうえさん", "あいどる"]
  print $ hoge ["12345", "67890", "12abc"]

heads :: [String] -> [Maybe Char]
heads xs = map f xs
  where
    f [] = Nothing
    f (x:_) = Just x

same :: Eq a => [a] -> Bool
same [] = True
same (x:xs) = all (== x) xs

hoge :: [String] -> [String]
hoge xs
  | same (heads xs) = hoge $ map tail xs
  | otherwise = xs